This invention relates to current regulation and more particularly to regulating lamp current to a constant level independent of the number of fluorescent lamps that are added to an electronic ballast.
Electronic ballast circuits are used in the operation of fluorescent lamps. An electronically controlled supply, such as a power factor correction circuit or other type of regulated supply, is used to provide the supply voltage to the electronic ballast circuit. Electronic ballast circuits are usually self-oscillating circuits and generally produce the high output voltage necessary for a fluorescent lamp to arc over. Once the fluorescent lamp arcs, a reactive impedance is used to limit, or ballast, the current through the lamp. This reactive impedance is reflected back into the oscillator circuitry causing the oscillator to shift in frequency. The greater the number of lamps added to the circuit, the greater the shift in frequency. A change from one to four lamps can create an oscillator frequency change of 25%, approximately a drop from 32 kHz to 24 kHz. This drop in frequency cuts the lamp current proportionately. Since most lamp manufacturers will not warrant their product for operating currents above 10% of the rated operating current, electronic ballasts are, in general, limited to lighting four lamps running at 80% of their rated current yielding only 80% of their light output.
The usual method to increase this light output is to increase the frequency of oscillation or increase the size of the lead-in, ballasting, capacitor to the fluorescent lamp. However, this method requires the swapping out of components and is difficult to maintain and to operate. Another alternative is to increase the supply voltage to the output circuit. However, this can be costly in reference to the high voltage components that may be necessary.
In view of the above, there is a need for a ballast circuit that can maintain a substantially constant lamp current regardless of the number of lamps on the circuit.